1. Field of the Invention
This invention relates to a drive circuit for an EL (Electroluminescent) element.
2. Description of the Related Art
There are examples at the present time of an EL element which is used as a backlight for a liquid crystal display provided on a small electronic appliance, such as a wristwatch. A drive circuit is required to generate an alternating current to drive an EL element of this kind. Although it has been a general practice to use a transformer as a voltage increasing element, this is not advantageous in encouraging size reduction.
FIG. 6 shows the type of drive circuit previously proposed. This circuit includes a voltage increasing circuit X1 to increase the voltage between a power terminal VDD and a power terminal VSS, and an H bridge circuit X3 to apply an output voltage of the voltage increasing circuit X1 to an EL element X2. The voltage increasing circuit X1 has a coil X4 and switch element X5 connected in series between the power terminal VDD and the power terminal VSS, and a diode X6 connected to a connection point between the coil X4 and the switch element X5. By intermittently turning on and off the switch element X5, an induced voltage is caused on the coil X4 and outputted through the diode X6. The H bridge circuit X3 has switch elements X7 and X8 connected in series between the diode X6 and the power terminal VSS, and switch elements X9 and X10 connected in series similarly between the diode X6 and the power terminal VSS, and an EL element X2 provided between a connection point between the switch elements X7 and X8 and a connection point between the switch elements X9 and X10, thereby charging and discharging the EL element in respective directions.
More explicitly, with reference to the timing graph of FIG. 7, if at time t0 the switch elements X7 and X10 are first turned on and then the switch element X5 begins intermittent on and off operation, the EL element X2 beings to be charged to a gradually increasing terminal-to-terminal voltage of the EL element X2 of approximately 100 V. Next, if at time t1 the switch element X5 is stopped from its intermittent on and off operation and the switch element X7 is turned off and the switch element X8 is turned on, then the voltage on the EL element X2 is discharged. Next, at time t2 the switch element X10 is turned off and the switch element X9 is turned on, and then the switch element X5 begins intermittent on and off operation. This causes the EL element X2 to be charged from the opposite terminal side thereof. Next, if at time t3 the switch element X5 is stopped from its intermittent on and off operation and the switch element X9 is turned off and the switch element X10 is turned on, then the voltage on the EL element X2 is discharged. By repeating the above operation, the EL element X2 is discharged in respective directions. Each switch element may be, for example, a bipolar transistor or a MOS transistor.